1. Field of the Invention
This invention relates to data transmission and in particular to data transmission in which a multi-level representation of a digital signal is transmitted.
2. Related Art
Digital data represents a signal that has been quantized in both time and amplitude. The digital data thus approximates the actual value of an analogue signal. If an analogue signal is digitized, the range of the analogue signal is divided into a number of levels e.g. 16, the analogue signal is sampled at set intervals and the appropriate level at that instant is determined. Since only 16 levels are used, the level which is closest to the actual level is chosen. A signal that is reconstructed from this digital data resembles but is not exactly the same as the original analogue signal.
In binary digital data, a signal is represented by 0 or 1, a 0 being a 0 V pulse and a 1 being a 5 V pulse for example. If a sample of an input signal has an amplitude greater than half of the maximum range, the signal sample is represented by a 1. A 0 will result from samples having an amplitude less than half. Thus a series of ones and zeros is produced.
To reproduce the original signal, a receiver needs to know the threshold between the two levels. This is usually done by the transmitter signalling to the receiver the maximum range of the signal, the number of levels and the spacing of the levels e.g. if they are linearly spaced etc. The receiver then determines the threshold(s) and decodes an incoming signal. In practice, in a multi-level system there are distortions in the signal due to the response of the network, namely overshoot and ringing. The instantaneous level of any received sample is not only dependent on the transmitted sample, but on the recent previously transmitted samples and possibly the subsequently transmitted sample or samples.